I feel your pain. I've pretty much never deleted a peice of work since i started 25 years ago. so finding work can take longer than doing the job sometimes!
You can change the number of items that are kept in the recent files list under the preferences, but this wont affect previously opened items beyond where you are currently at i'm afraid.
Best bet is probably to just set up some smart search folders in Finder (Mac) or explorer? on PC.
you just need to search from the top level drive so it searches the whole computer / all drives and volumes etc, then add some criteria. file type .ai / date last modified within 'x' number of days / weeks etc. and you should be able to whittle it down a bit.
you sometimes need to swap between date last modified, and date created depending on whether your computer is playing ball, as it will sometimes only show files that have been created and then modified and saved again, sometime it will do the opposite.
The file usually turns up in the 'wrong' folder having been accidentally saved to the location of the previous project.